The present invention relates to a method for forming a pattern and to a photoresist used therein and, more particularly, to a method for forming a pattern of a non-light-emitting black material on the inner surface of a faceplate of a color picture tube or a cathode ray tube and to a photoresist used therein.
A photoresist is used for forming a pattern of a non-light-emitting black material on the inner surface of a faceplate of a color picture tube. The photoresist used for this purpose must satisfy many requirements; it must be water-soluble, have high sensitivity, have high adhesion strength, have no dark reaction, have no thermal fog, have good developing characteristics and allow easy removal of a photocured photoresist film.
In general, a phosphor screen of a color picture tube is prepared by a series of steps presented below including a step for forming a pattern of a non-light-emitting black material:
(1) Resist coating: A photoresist is coated on the inner surface of a faceplate, and is then dried to form a photoresist film.
(2) Exposure: A shadow mask is formed which has holes for allowing passage of an electron beam therethrough. The light is radiated onto the photoresist film through the shadow mask to form photocured dots for holes for embedding primary phosphors for R (red), G (green), and B (blue).
(3) Developing: After removing the shadow mask, the photoresist film is rinsed with water to leave the photocured dots and remove the remainder of the film.
(4) Formation of non-light-emitting black material film: A colloidal black carbon suspension is coated on the inner surface of the faceplate and is dried.
(5) Removal: A removal solution is applied to remove the photocured dots of the photoresist, thus formating the holes in the non-light-emitting black material film.
(6) Layers of primary phosphors for R, G and B are formed on the non-light-emitting black material film including the holes.
Photoresists to be used for forming a pattern of a non-light-emitting black material on the inner surface of a faceplate of a color picture tube are conventionally known by several names, such as a PVA-ADC-type photoresist consisting of polyvinyl alcohol (PVA) and ammonium dishromate (ADC) or sodium dichromate; a so-called PVA-diazocompound-type photoresist consisting of PVA and a water-soluble diazocompound; a so-called PVP-azidocompound-type photoresist consisting of polyvinyl pyrrolidone (PVP) and a water-soluble bisazidocompound; and so on.
However, these conventional photoresists are subject to various drawbacks; none of them satisfies all the requirements as described above when they are used to form a pattern of a non-light-emitting black material on the inner surface of a faceplate of a color picture tube.
Although a PVA-ADC-type photoresist is highly sensitive, its sensitivity to the light is subject to large changes depending upon the exposure. Control of the size of the holes to be formed in the non-light-emitting black material film is difficult. Dark reaction and/or thermal blushing may occur. In order to avoid these problems, the working conditions must be carefully set and must be well controlled.
A PVA-diazocompound-type photoresist has a good adhesion strength. However, it has poor developing characteristics and poor sensitivity, providing an impractical photoresist.
Although a PVP-azidocompound-type photoresist has good developing characteristics, it has poor adhesion strength. A pattern may not be formed with good precision unless light of more than a predetermined intensity is radiated thereon.